For a document image obtained through scanning or photographing, e.g. a document image scanned by an overhead scanner, since a document with fluctuations on a surface thereof is illuminated by a light source such as an LED of the scanner, the scanned image obtained possibly has a shadow. Due to the existence of the shadow, the quality of the image is deteriorated. Therefore, it is necessary to remove the shadow such that a background of the image is uniform.
The currently-used method of removing a shadow is to adjust, according to a luminance image estimated from a document image, precisely speaking, a luminance component image of the document image, an image intensity of a document image (i.e., the luminance component image), so as to remove the shadow, thereby obtaining a document image with a uniform background. Hereinafter, the fundamental principle of removing a shadow will be described in combination with equations (1) and (2). As shown in equation (1):IY=LY·RY  (1)
where IY represents an input image, LY represents a luminance image, RY represents a reflected image, and the subscript Y represents that the images are luminance component images.
Therefore, an image with a uniform background can be obtained from equation (2):
                                          I            ~                    Y                =                              cR            Y                    =                                    cI              Y                                                      L                Y                            +              ɛ                                                          (        2        )            
where c is a constant, which means an average image gray scale; and ε is a very small constant, serving to prevent a denominator from being 0.
FIG. 1 is a view illustrating a luminance component image of a document image obtained through scanning ((a) in FIG. 1), a luminance image estimated according to the luminance component image ((b) in FIG. 1) and an adjusted luminance component image after removal of a shadow ((c) in FIG. 1). It follows that obtaining an accurate luminance image performs the function of adjusting the luminance component image accurately, so as to perform shadow removal on the document image. At present, more and more attention is being drawn to the problem as to how to obtain a more accurate luminance image.